1. Field of the Invention
This invention relates generally to meta, para-cresol mixtures and more particularly to a process for the reduction of the content of the impurity ortho-ethylphenol from a mixture of meta-, para-cresol isomers.
2. Description of the Prior Art
The cresols, CH.sub.3 C.sub.6 H.sub.4 OH, exist in three isomeric forms. The ortho-isomer has a boiling point such that it can be readily separated from the m- and p- isomers by distillation. However, the meta- and para- isomers have boiling points which are too close together for the two isomers to be separated by fractional distillation. Consequently, ortho-cresol is marketed as the isomer, itself, but m- and p-cresols are marketed generally as a mixture under a specification of 2.degree., 3.degree. or 5.degree. C. including the true boiling points of m- and p-cresol.
The major source of commercial cresols is from the tar produced by the coking of coal. Another source is from cracked petroleum distillates. However, the mixture of m- and p-cresols that is derived from petroleum sources tends to be contaminated with o-ethylphenol.
In the two major outlets that use a mixture of the cresols, m,p-cresol, as a starting material, the impurity o-ethylphenol is undesirable. When the cresol mixture is used in the manufacture of tricresyl phosphates, which are used as plasticizers and heat transfer media, any resultant phosphates made in part or in toto from ortho-alkylphenols such as o-cresol and o-ethylphenol are poisonous to mammals. The other major use for m,p-cresol is as starting material for the manufacture of butylated derivatives which are antioxidants such as butylated hydroxytoluene (BHT) or intermediates for antioxidants, and impurities here should be minimized. As is shown in the table of boiling points of these derivatives below, even the t-butylated o-ethylphenols boil at temperatures so close to those of the butylated derivatives of m,p-cresols that the isolation of these compounds is difficult.
Typical of commercially available mixtures of m- and p-cresols from a petroleum source is a material which is sold as M-P 88, a tar acid blend produced by the Productol Chemical Division of Ferro Corporation, and which contains a minimum of 88% m,p-cresol, about 0.5-6% of o-ethylphenol (OEP) and lesser amounts of o-cresol, 2,4- , 2,5-xylenol and phenol. The difficulty of removing o-ethylphenol from m,p-cresol by fractional dishtillation may be seen by examining the boiling points expressed in degrees centigrade at various pressures expressed in millimeters of mercury absolute:
______________________________________ Boiling Point Compound 50 mm 100 mm 760 mm ______________________________________ m,p-cresol 120 137 202 2-ethylphenol 121 139 204 ______________________________________
The patent and chemical literature describe numerous processes for separating the pure meta and para isomers from the meta-para mixture. A preferred process is an alkylation procedure. According to this process the mixture is alkylated with isobutylene, the di-tert-butylated products are separated by fractional distillation, and the butylated products are debutylated. The products recovered are m-cresol, p-cresol and isobutylene.
The foregoing process is not feasible for the separation of the o-ethylene derivative from the other. The following table showing the boiling points of the t-butylated derivatives of m,p-cresols and o-ethylphenol at various pressures demonstrates why it is so difficult to separate the ortho-ethylphenol by means of a process that is designed to produce pure di-t-butylated cresol isomers.
______________________________________ Boiling Point .degree.C. Pressures in mm Hg Compound 50 mm 100 mm 760 mm ______________________________________ 2-t-butyl-p-cresol (MBPC) 149 168 238 6-t-butyl-m-cresol (MBMC) 153 172 243 6-t-butyl-2-ethylphenol (OBOEP) 152 4-t-butyl-2-ethylphenol (PBOEP) 168 187 254 2-6-di-t-butyl-p-cresol (BHT) 170 190 266 4-6-di-t-butyl-2-ethylphenol 178 (DBOEP) 4,6-di-t-butyl-m-cresol (DBMC) 192 211 282 ______________________________________